Modern vehicles are often equipped with electrically actuated parking brakes, which are also referred to as automatic parking brakes (APBs). Parking brakes of this kind usually encompass an operating element, such as, e.g., a button, with which the parking brake can be locked or released. Upon an actuation of the button, a control unit connected thereto detects the parking brake request and correspondingly applies control to an actuator, for example to an electric motor, to engage or release the parking brake.
In addition, vehicles are often also equipped with braking functions that automatically hold the vehicle at a standstill with no need for the driver to actuate the brake pedal for that purpose. Known braking functions are, for example, automatic vehicle hold (AVH) or adaptive cruise control (ACC) plus stop-and-go. In the case of the AVH function, the brake pressure present at the wheel brakes is retained, and the vehicle is automatically held at a standstill; with the ACC plus stop-and-go function, in an appropriate traffic situation the vehicle is decelerated to a standstill, and automatically held, with no driver intervention.
To allow the brake pressure at the wheel brakes to be automatically held, a valve of the braking system, usually the so-called switchover valve, is closed. This requires that a specific valve current be applied to the valve. The switchover valves cannot, however, have current flowing through them for an arbitrarily long time, since otherwise they overheat. With conventional valves, the maximum capacity at a holding pressure of 45 bar is approximately ten minutes. The responsibility for holding the vehicle is therefore transferred, at the latest before the valves overheat, to a parking brake with which the vehicle can be held in zero-energy fashion. A switchover therefore occurs from the hydraulic brake to the parking brake.
In the switchover phase or transition phase, control is applied to the locking devices of the parking brake by a control unit so that they lock. Coincidentally in time or thereafter, the brake pressure present at the wheel brakes is reduced. As soon as the hydraulic brake pressure at the wheels is reduced, the vehicle as a rule is then braked only by those wheels on which the parking brake is located. These are usually the rear wheels. On steeper slopes and/or on a slippery surface, it is possible as a result for the vehicle abruptly to start slipping, and then to move downslope with the rear axle blocked. In this case the driver would need to recognize the critical situation and push the brake pedal in order to brake the vehicle. Since the transition to the parking brake does not occurs until a few minutes after coming to a stop, however, the driver as a rule is no longer attentive and can react only in delayed fashion to the fact that the vehicle is slipping away. By the time the driver has recognized the situation and actuated the brake pedal, the vehicle may already have moved a relatively long distance downhill. The switchover to the parking brake, and the abrupt slippage of the vehicle associated therewith, can therefore result in a danger to the driver and occupants, or to other traffic participants.